Unit 7 Reflection!

http://buphy.bu.edu/~duffy/PY106/2e.GIF

This unit was all about energy electricity! We learned how lightnings work, how are houses are wired, why energy flows, what voltage is and much more!
One of the first things we established and that helped us through the entire unit was that opposite charges attract each other, and like charges repel. Charges can never be destroyed, only transferred and electrons (negative charges) can move better than protons (positive charges) because they are not bound to the nucleus as the protons are. After learning the relationship of charges we establish what we call Coulomb's Law represented by the equation:

F = (K q1q2) /d²

Coulomb's law means that the closer something is to another, the greater the force between them is - Distance is inversely proportional to force. 

Electric fields was a concept that I did not understand in the beginning of this unit. I latter found out that it was simply the area where a force can be felt. The complicated pictures with arrows only symbolized where a positive test charge would move. The equations for electric field are:

E = F/q  

 and

E = (K q1q2)/d²

Another concept learned was the difference between a insulator and a conductor. Conductor's electrons are free to move and an insulator's electrons are closely bound to particular atoms. 

Now we can put together Coulomb's law, the relationship of charges and the concept of insulator and conductor we can explain why a balloon can stick to a wall for example.

Here is why the boy can stick the balloon to the wall after rubbing it to his sister's hair. When he rubs the balloon in the baby's head the balloon picks up electrons from the hair and becomes negatively charged (by friction). After that, he puts the balloon on the wall. Because the wall is an insulator the electrons of the balloon will not be stolen by the wall, but the charges will rearrange in a way that the positive charges will move close to the negatively charged balloon, and the negative charges of the wall will move as far away from it as possible. This process of rearrangement of charges is called polarization. The illustration bellow shows what the charges of the balloon and the wall look before and after polarization. You might be wondering why it sticks if there is also a force of repulsion between the like charges of the balloon and wall. Here is when Coulomb's law comes in handy! Coulomb's law says that distance is inversely proportional to force so since the opposite charges are closer together, the force of attraction is stronger than the force of repulsion, therefore the balloon sticks to the wall!

http://www.teachengineering.org/collection/cub_/lessons/cub_images/
cub_electricity_lesson02_activity1_fig3.jpg
Teach Engineering

The same idea that explains a balloon sticking to the wall can explain why the comb of the video bellow can attract the water. The comb is negatively charged and the water is neutral. The charges of the water become polarized and because of Coulomb's the attractive force wins once again!

There are many ways of charging an object, one of them we already mentioned before and that is friction. When something rubs against another electrons tend to be transfered therefore charging it through friction (like the balloon that picked up electrons from the hair). Another way of charging an object is by induction; induction happens when something is charges without touch.An example of induction is the formation of lightings. Lightning is caused by a combination of facts. All starts when clouds rubb against each other and the bottom of the clouds become negatively charged by friction. The negative charges of the clouds attract the positive charges of the ground by induction, because opposite charges attract. Like charges repel, therefore the negative charges of the ground will go deep into the ground. The attraction of the electrons of the clouds and the protons of the ground is so great that the charges create a path so that they can travel from cloud to ground and ground to clouds. This exchange of charges emmits the light we see and call lightning.




http://www.outdoored.com/anm/templates/default.aspx?a=1807&template=print-article.htm



We can't avoid having lightning strikes, but how can you protect your house from them? Lightning rods can protect your house of the damish lightnings can cause. For an unknown reason, positive charges build up on sharp points and that is exactly what lighting rods have. The rod will be placed on your roof and because of the positive charges that build up on it when there is a lightning the electrons from the clouds will be attracted to those charges. When the lightining strikes the rod the charges will flow through a metal wire that will ground the charges protecting your house from it.

From the begging of the unit we learned that for charge to flow there needs to be a complete circuit, and later on we learned that for current to flow there also needs to be an electric potential difference. The voltages from one place to the othere needs to be different.


A capacitor is what makes the potential difference to create a circuit. It builds up electric potential difference by having two oppositely charged plates (they are attracted to each other). This is the mechanics behind a defibrillator and the flash on a camera. In a defibrillator the electric potential difference is build between the plates, and the circuit is complete when they touch the patient's body. The energy flows through the person and the defibrillator needs to charge, so that the electric potential difference can be built up again.

http://blog.mobileframe.com/wp-content/uploads/defibrillator-paddles.gif
mobileframe

When talking about how current flows we learned that voltage doesn't move through a circuit but "pushes" current through. We also learned about resistance it is basically how much an appliance oposes the passage of current through a circuit. Resistance is not the only determinant of resistance, here are some other factors:

• Length (the longest the more resistance)
• Width (The wider the less resistance)
• Temperature (The hotter the more resistance)
• Material

 After learning this deffinitions we found out that:

Current = Voltage / Resistance



With the equation above is easy to analyse the affect of resistance in current. Now you can explain why light bulbs blow right after they are turned on because you know the effect of a cold temperature to resistance and the effect of resistance to current.

Power = Current x Voltage

This equation relates to watts. You will probably come across a light bulb that has a label saying how many watts it uses. Withe the equation above you can find how much current passes through the circuit for each light bulb therefore you can find how bright they are. 

http://www.mos.org/sln/toe/history.html

Voltage = Electric Potential Energy / Charge

We talked a lot about this equation when we tried to figure it out why a van de Graaff generator, that has a high voltage, doesn't hurt you, but an outlet that has a much lower voltage does. We stablished that what hurts you is not volte but energy. We used the equation above to plug in the voltage and charge in order to find how much energy the outlet, and the generator had. We found that the generator has a small energy and the outlet has a big one therefore it is safe to touch the generator but not the outlet. 



http://www.instructables.com/id/Operation-Game/step16/About-Parallel-and-Series-Circuits/
Instructables



The last thing we learned about was parallel and series circuits. In the image above you can see an example of series circuit (left) and a parallel circuit (right). The main difference between the circuits is the effect of switching one of the appliances off. In the series circuit all appliances would go off if one got of'; in parallel the other appliances would keep on working. Because of that, our houses are wired in parallel. We also discovered in a lab that a parallel circuit has more "lanes" and therefore the resistance is lower and since 
I = V/R the current is greater. Series has more "stoplights" therefore the resistance is higher and current is lower. We said earlier in the post that current is directly proportional to the brightness/efficiency of an appliance, therefore the bulbs in parallel would be brighter than the ones in series. Another important thing to remember is the role of fuses in a circuit. Fuses are wired in series and they are sensitive to the amount of current in that circuit, if the current gets to high so that it becomes dangerous, the filament of the fuse will break causing the current to stop flowing. That might be extremely annoying if it happens in your house, but is saving you from a possible fire. But why would the current go up? In a parallel current the more appliances you plug in the system the higher the current will get! In a series circuit on the other hand, the total current remains the same and the individual goes down. 


Reflection:


What I found the most difficult in this unit was connecting one concept with the other. Because the unit was long, it was hard to remember equations and to what they related to. The idea of voltage, electric potential energy, and electric potential sometimes seemed to be the same exact thing. The way I overcame this problems was by making many mistakes in homework assignments and quizzes, and looking back to my notes after that and finding out what I did wrong. Coming in for help and asking questions also was a great tool to finally understand this sometimes confusing topics.